Static random access memory (sram) device for improving electrical characteristics and logic device including the same

ABSTRACT

A static random access memory (SRAM) device includes a circuit element that includes a first inverter having a first load transistor and a first drive transistor and a second inverter having a second load transistor and a second drive transistor. Input and output nodes of the first inverter and the second inverter are cross-connected to each other. A first transfer transistor is connected to the output node of the first inverter, and a second transfer transistor is connected to the output nodes of the second inverter. Each of the first and second load transistors, the first and second drive transistors, and the first and second transfer transistors includes a transistor having multi-bridge channels. At least one of the first and second load transistors, the first and second drive transistors, and the first and second transfer transistors includes a transistor having a different number of multi-bridge channels from the other transistors.

CROSS-REFERENCE TO RELATED APPLICATION

This U.S. non-provisional patent application is a continuation of U.S.patent application Ser. No. 15/246,526, filed on Aug. 24, 2016, whichclaims the benefit of Korean Patent Application No. 10-2015-0171435,filed on Dec. 3, 2015, in the Korean Intellectual Property Office, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The inventive concept relates to a static random access memory (SRAM)device, and more particularly, to a SRAM device having improvedelectrical characteristics, and a logic device that includes the SRAMdevice.

An SRAM device exhibits lower power consumption and faster operatingcharacteristics than a dynamic random access memory (DRAM) device, andhas widely been applied to cache memory devices of computers or portableelectronic products. Nevertheless, it is still necessary to improveelectrical characteristics that are important to the operation of a SRAMdevice.

SUMMARY

The inventive concept provides a static random access memory (SRAM)device that has improved electrical characteristics.

The inventive concept also provides a logic device including the SRAMdevice.

According to an aspect of the inventive concept, there is provided amemory device including a circuit element including a first inverterincluding a first load transistor and a first drive transistor and asecond inverter including a second load transistor and a second drivetransistor, wherein input and output nodes of the first inverter and thesecond inverter are cross-connected to each other, a first transfertransistor connected to the output node of the first inverter, and asecond transfer transistor connected to the output node of the secondinverter.

Each of the first and second load transistors, the first and seconddrive transistors, and the first and second transfer transistorsincludes a transistor having multi-bridge channels. At least one of thefirst and second load transistors, the first and second drivetransistors, and the first and second transfer transistors includes atransistor having a different number of multi-bridge channels from theother transistors.

According to another aspect of the inventive concept, there is provideda memory device including first to fourth multi-bridge channelstructures arranged in a second direction and sequentially spaced apartfrom one another in a first direction that is substantiallyperpendicular to the second direction, a first gate structure arrangedin the first direction, the first gate structure surrounding the firstand second multi-bridge channel structures, first and second source anddrain regions located in first and second multi-bridge channelstructures on respective sides of the first gate structure, a secondgate structure arranged in the first direction and spaced apart from thefirst gate structure in the second direction, the second gate structuresurrounding the first multi-bridge channel structure, third source anddrain regions located in the first multi-bridge channel structure onrespective sides of the second gate structure, a third gate structurespaced apart from the first gate structure in the second direction andspaced apart from the second gate structure in the first direction, thethird gate structure surrounding the third and fourth multi-bridgechannel structures, fifth source and drain regions located in the thirdand fourth multi-bridge channel structures on respective sides of thethird gate structure, a fourth gate structure spaced apart from thefirst gate structure in the first direction, the fourth gate structuresurrounding the fourth multi-bridge channel structure, and sixth sourceand drain regions located in the fourth multi-bridge channel structureon respective sides of the fourth gate structure.

According to another aspect of the inventive concept, there is provideda memory device including an SRAM forming region including an SRAMdevice and a logic region configured to process data. The SRAM deviceincludes a first inverter including a first load transistor and a firstdrive transistor, a second inverter including a second load transistorand a second drive transistor, a first transfer transistor connected toan output node of the first inverter, and a second transfer transistorconnected to an output node of the second inverter. At least one of thefirst and second load transistors, the first and second drivetransistors, and the first and second transfer transistors includes atransistor having a different number of multi-bridge channels from theother transistors.

According to another aspect of the inventive concept, a memory devicecomprises a latch circuit, a first transfer transistor and a secondtransfer transistor. The latch circuit comprises a first inverter and asecond inverter. The first inverter comprises a first input node, afirst load transistor, a first drive transistor and a first output node.The second inverter comprises a second input node, a second loadtransistor, a second drive transistor and a second output node. Thefirst output node may be electrically connected to the second input nodeand the second output node may be electrically connected to the firstinput node. The first transfer transistor may be connected to the firstoutput node. The second transfer transistor may be connected to thesecond output node. Each of the first and second load transistors, thefirst and second drive transistors, and the first and second transfertransistors may comprises a multi-bridge channel transistor, and atleast one of the first and second load transistors, the first and seconddrive transistors, and the first and second transfer transistors maycomprise a multi-bridge channel transistor having a number ofmulti-bridge channels that is different from a number of multi-bridgechannels of the other transistors.

According to another aspect of the inventive concept, a memory devicecomprises a first to a fourth multi-bridge channel structures extendingin a first direction and sequentially spaced apart from one another in asecond direction that is substantially perpendicular to the firstdirection; a first gate structure extending in the second direction, thefirst gate structure surrounding the first and second multi-bridgechannel structures; a first source region and a first drain regionlocated in the first multi-bridge channel structure on respective sidesof the first gate structure; a second source region and a second drainregion located in the second multi-bridge channel structure onrespective sides of the first gate structure; a second gate structureextending in the second direction and spaced apart from the first gatestructure in the first direction, the second gate structure surroundingthe first multi-bridge channel structure; a third source region and athird drain region located in the first multi-bridge channel structureon respective sides of the second gate structure; a third gate structurespaced apart from the first gate structure in the first direction andspaced apart from the second gate structure in the second direction, thethird gate structure surrounding the third and fourth multi-bridgechannel structures; a fourth source region and a fourth drain regionlocated in the third multi-bridge channel structure on respective sidesof the third gate structure; a fifth source region and a fifth drainregion located in the fourth multi-bridge channel structure onrespective sides of the third gate structure; a fourth gate structurespaced apart from the third gate structure in the second direction andspaced apart from the first gate structure in the second direction, thefourth gate structure surrounding the fourth multi-bridge channelstructure; and a sixth source and a sixth drain region located in thefourth multi-bridge channel structure on respective sides of the fourthgate structure. Each of the first to fourth multi-bridge channelstructures may comprise a plurality of nano-bridges as channels in whichthe plurality of nano-bridges may be stacked apart from one another in athird direction that is substantially perpendicular to a plane definedby the first direction and the second direction, and at least one of thefirst to fourth multi-bridge channel structures may comprise a number ofnano-bridges that is different from a number of nano-bridges of theother multi-bridge channel structures.

According to still another aspect of the inventive concept, a memorydevice comprises a logic region and a static random access memory (SRAM)region that includes an SRAM device. The SRAM device may comprise afirst inverter and a second inverter. The first inverter may comprise afirst input node, a first load transistor, a first drive transistor anda first output node. The second inverter may comprise a second inputnode, a second load transistor, a second drive transistor and a secondoutput node. The first output node may be electrically connected to thesecond input node and the second output node may be electricallyconnected to the first input node. At least one of the first and secondload transistors, the first and second drive transistors, and the firstand second transfer transistors may include a transistor having a numberof multi-bridge channels that is different from a number of multi-bridgechannels of the other transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the inventive concept will be more clearly understoodfrom the following detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is an equivalent circuit diagram of a static random access memory(SRAM) device according to an embodiment;

FIG. 2 depicts an embodiment of an example layout of an SRAM device thatincludes a transistor having multi-bridge channels of FIG. 1;

FIG. 3 depicts an embodiment of an example layout of an SRAM device thatincludes the multi-bridge channel structures and gate structures of FIG.2;

FIG. 4 depicts a perspective view of transistors having multi-bridgechannels used for an SRAM device according to an embodiment;

FIG. 5 depicts a cross-sectional view taken long a line XA-XA′ of FIG.4;

FIG. 6 depicts a cross-sectional view taken along lines YA-YA′ andYB-YB′ of FIG. 4;

FIGS. 7A and 7B depict cross-sectional views of transistors havingmulti-bridge channels used for an SRAM device according to anembodiment;

FIG. 8 depicts a cross-sectional view of transistors having multi-bridgechannels used for an SRAM device according to an embodiment;

FIGS. 9 and 10 depict cross-sectional views of transistors havingmulti-bridge channels used for an SRAM device according to anembodiment;

FIGS. 11A to 20A and 11B to 20B depict stages of a method ofmanufacturing transistors according to an embodiment;

FIG. 21 is a flow diagram of a method of manufacturing transistorsaccording to an embodiment;

FIG. 22 depicts a schematic diagram of a logic device including an SRAMdevice according to an embodiment;

FIG. 23 depicts a schematic diagram of a card including an SRAM deviceaccording to an embodiment;

FIG. 24 depicts a schematic block diagram of an electronic circuit boardincluding an SRAM device according to an embodiment;

FIG. 25 depicts a schematic block diagram of an electronic systemincluding an SRAM device according to an embodiment;

FIG. 26 depicts a schematic diagram of an electronic system including anSRAM device according to an embodiment; and

FIG. 27 depicts a schematic perspective view of an electronic deviceincluding an SRAM device according to an embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The inventive concept is disclosed by describing the followingembodiments or combinations of embodiments. Accordingly, it should beunderstood that the inventive concept should not be construed as limitedto embodiments or combinations of embodiments set forth herein.

FIG. 1 is an equivalent circuit diagram of an SRAM device 10 accordingto an embodiment.

Specifically, FIG. 1 depicts an embodiment of one static random accessmemory (SRAM) cell of the SRAM device 10 for brevity. In one embodiment,the SRAM device 10 may include a plurality of SRAM cells arranged as anarray.

The SRAM device 10 may include bit lines BL and/BL, a word line WL, andsix transistors, for example, first and second transfer transistors PG1and PG2, first and second load transistors PU1 and PU2, and first andsecond drive transistors PD1 and PD2. The first and second transfertransistors PG1 and PG2 may be referred to as pass transistors. Thefirst and second load transistors PU1 and PU2 may be referred to aspull-up transistors. The first and second drive transistors PD1 and PD2may be referred to as pull-down transistors.

The first and second load transistors PU1 and PU2 may include p-typemetal oxide semiconductor (PMOS) transistors, and the first and secondtransfer transistors PG1 and PG2 and the first and second drivetransistors PD1 and PD2 may include n-type MOS (NMOS) transistors.

Gates G(PG1) and G(PG2) of the first and second transfer transistors PG1and PG2 may be electrically connected to the word line WL. Drain regionsD(PG1) and D(PG2) of the first and second transfer transistors PG1 andPG2 may respectively be electrically connected to a pair of bit lines BLand/BL. Source regions S(PU1) and S(PU2) of the first and second loadtransistors PU1 and PU2 may be connected to a first power supply voltageVdd, and source regions S(PD1) and S(PD2) of the first and second drivetransistors PD1 and PD2 may be connected to a second power supplyvoltage GND. The first power supply voltage Vdd may be a power supplyvoltage, and the second power supply voltage GND may be a ground powersupply voltage.

The first load transistor PU1 and the first drive transistor PD1 mayform a first inverter INV1, and the second load transistor PU2 and thesecond drive transistor PD2 may form a second inverter INV2. A sourceregion S(PG1) of the first transfer transistor PG1, a drain regionD(PU1) of the first load transistor PU1 and a drain region D(PD1) of thefirst drive transistor PD1 may be electrically connected in common to afirst node N1. The first transfer transistor PG1 may be connected to anoutput node (first node) N1 of the first inverter INV1.

A source region S(PG2) of the second transfer transistor PG2, a drainregion D(PU2) of the second load transistor PU2 and a drain regionD(PD2) of the second drive transistor PD2 may be electrically connectedin common to a second node N2. The second transfer transistor PG1 may beconnected to an output node (second node) N2 of the second inverterINV2.

A gate G(PU1) of the first load transistor PU1 and a gate G(PD1) of thefirst drive transistor PD1 may be electrically connected in common to afirst input node I1 and to the second node N2, and form a first latchcircuit. A gate G(PU2) of the second load transistor PU2 and a gateG(PD2) of the second drive transistor PD2 may be electrically connectedin common to a second input node 12 and to the first node N1, and form asecond latch circuit.

The input nodes I1 and 12 and the output nodes N1 and N2 of the firstinverter INV1 and the second inverter INV2 may be cross-connected to oneanother. That is, the output node N1 of the first inverter INV1 may beconnected to the input node 12 of the second inverter INV 2. The outputnode N2 of the second inverter INV2 may be connected to the I1 of thefirst inverter INV2.

Thus, the SRAM device 10 may comprise a circuit element CE, whichincludes the first inverter INV1, the second inverter INV2, and aninterconnection line configured to connect the input nodes I1 and 12 andthe output nodes N1 and N2 of the first inverter INV1 and the secondinverter INV2. The circuit element CE may be a flip-flop circuit or alatch circuit that serves as an information accumulator configured tostore one bit of information.

A circuit operation the circuit CE will now be briefly described. If thefirst node N1 of the first inverter INV1 is at a high electric potentialH, the second drive transistor PD2 is turned on and the second node N2of the second inverter INV2 may be at a low electric potential L.Accordingly, the first drive transistor PD1 may be turned off so that afirst node N1 may be maintained at the high electric potential H. Thatis, states of the first node N1 and the second node N2 may be maintainedby the latch circuit that is configured by cross-coupling the first andsecond inverters INV1 and INV2, and information may be retained duringthe application of the first power supply voltage Vdd to the circuitelement CE.

Additionally, if the word line WL is at a high electric potential H, thefirst and second transfer transistors PG1 and PG2 may be turned on, andthe latch circuit may be electrically connected to the bit lines BLand/BL. Thus, an electric potential state, that may be either H or L, ofeach of the first and second nodes N1 and N2 may be transmitted ortransferred to the bit lines BL and/BL and read as information containedin the SRAM cell of the SRAM device 10. To write information to the SRAMcell, information of the bit lines BL and/BL may be transmitted to thefirst and second nodes N1 and N2 by setting the word line WL to the highelectric potential H and turning on the first and second transfertransistors PG1 and PG2. Accordingly, the SRAM device 10 may perform aread operation and a write operation.

In the SRAM device 10 according to the present embodiment, each of thefirst and second load transistors PU1 and PU2, the first and seconddrive transistors PD1 and PD2, and the first and second transfertransistors PG1 and PG2 may include a transistor having multi-bridgechannels to improve electrical characteristics during theabove-described circuit operation.

In a transistor having the multi-bridge channels, a plurality ofchannels may be vertically stacked apart from one another as describedbelow.

Since a transistor having the multi-bridge channels is capable ofreducing a short-channel effect, reducing a narrow-width effect, andreducing an area occupied by source and drain regions, the transistorhaving the multi-bridge channels may be advantageous to increasingintegration density. Also, a uniform source/drain junction capacitancemay be maintained irrespective of a position of a channel, so ahigh-speed highly reliable device may be manufactured.

In addition, in the SRAM device 10 according to the present embodiment,at least one of the first and second load transistors PU1 and PU2, thefirst and second drive transistors PD1 and PD2, and the first and secondtransfer transistors PG1 and PG2 may include a transistor having anumber of multi-bridge channels that is different from the number ofmulti-bridge channels of the other transistors.

A transistor having a multi-bridge channel and transistors havingdifferent numbers of multi-bridge channels will be described in detaillater.

FIG. 2 depicts an embodiment of an example layout of an SRAM device 10that includes a transistor having multi-bridge channels of FIG. 1. FIG.3 depicts an embodiment of an example layout of an SRAM device thatincludes the multi-bridge channel structures and gate structures of FIG.2.

Specifically, the SRAM device 10 may include first to fourthmulti-bridge channel structures MBCS1, MBCS2, MBCS3 and MBCS4, which maybe arranged to extend in a second direction (e.g., the Y direction) andsequentially arranged apart from one another in a first direction (e.g.,the X direction) that is substantially perpendicular to the seconddirection. The first to fourth multi-bridge channel structures MBCS1,MBCS2, MBCS3 and MBCS4 may be formed on an N-type well region NW and aP-type well region PW.

A first gate structure GS1 may be arranged to extend in the firstdirection on the first and second multi-bridge channel structures MBCS1and MBCS2. As described below, the first gate structure GS1 may surroundthe first and second multi-bridge channel structures MBCS1 and MBCS2.

First source and drain regions 51 and D1 may be formed in the firstmulti-bridge channel structure MBCS1 on respective sides of the firstgate structure GS1. The first source and drain regions 51 and D1 may beformed in the second direction (or Y direction) from each other in themulti-bridge channel structure MBCS1. The first gate structure GS1, thefirst multi-bridge channel structure MBCS1, and the first source anddrain regions 51 and D1 may form a first drive transistor PD1. The firstdrive transistor PD1 may form an NMOS transistor.

Second source and drain regions S2 and D2 may be formed in the secondmulti-bridge channel structure MBCS2 on respective sides of the firstgate structure GS1. The first source and drain regions S2 and D2 may beformed in the second direction (i.e., Y direction) from each other inthe multi-bridge channel structure MBCS2. The first gate structure GS1,the second multi-bridge channel structure MBCS2, and the second sourceand drain regions S2 and D2 may form a first load transistor PU1 _(—)The first load transistor PU1 may form a PMOS transistor.

A second gate structure GS2 may be spaced apart from the first gatestructure GS1 in the second direction and located on the firstmulti-bridge channel structure MBCS1 in the first direction. Asdescribed below, the second gate structure GS2 may surround the firstmulti-bridge channel structure MBCS1.

Third source and drain regions S3 and D3 may be formed in the firstmulti-bridge channel structure MBCS1 on respective sides of the secondgate structure GS2. The third source and drain regions S3 and D3 may beformed in the second direction from each other. The second gatestructure GS2, the first multi-bridge channel structure MBCS1, and thethird source and drain regions S3 and D3 may form a first transfertransistor PG1. The first transfer transistor PG1 may form an NMOStransistor.

A third gate structure GS3 may be spaced apart from the first gatestructure GS1 in the second direction, spaced apart from the second gatestructure GS2 in the first direction, and may be located on the thirdand fourth multi-bridge channel structures MBCS3 and MBCS4. As describedbelow, the third gate structure GS3 may surround the third and fourthmulti-bridge channel structures MBCS3 and MBCS4.

Fourth source and drain regions S4 and D4 may be formed in the thirdmulti-bridge channel structure MBCS3 on respective sides of the thirdgate structure GS3. The fourth source and drain regions S4 and D4 may beformed in the second direction from each other in the third multi-bridgechannel structure MBCS3. The third gate structure GS3, the thirdmulti-bridge channel structure MBCS3, and the fourth source and drainregions S4 and D4 may form a second load transistor PU2. The second loadtransistor PU2 may form a PMOS transistor.

Fifth source and drain regions S5 and D5 may be formed in the fourthmulti-bridge channel structure MBCS4 on respective sides of the thirdgate structure GS3. The fifth source and drain regions S5 and D5 may beformed in the second direction from each other in the fourthmulti-bridge channel structure MBSC4. The third gate structure GS3, thefourth multi-bridge channel structure MBCS4, and the fifth source anddrain regions S5 and D5 may form a second drive transistor PD2. Thesecond drive transistor PD2 may form an NMOS transistor.

A fourth gate structure GS4 may be spaced apart from the first gatestructure GS1 in the first direction and may be located on the fourthmulti-bridge channel MBCS4. As described below, the fourth gatestructure GS4 may surround the fourth multi-bridge channel structureMBCS4.

Sixth source and drain regions S6 and D6 may be formed in the fourthmulti-bridge channel structure MBCS4 on respective sides of the fourthgate structure GS4. The sixth source and drain regions S6 and D6 may beformed in the second direction from each other in the fourthmulti-bridge channel structure MBCS4. The fourth gate structure GS4, thefourth multi-bridge channel structure MBCS4, and the sixth source anddrain regions S6 and D6 may form a second transfer transistor PG2. Thesecond transfer transistor PG2 may form an NMOS transistor.

The first multi-bridge channel structure MBCS1 may be electricallyconnected to the second multi-bridge channel structure MBCS2 via a firstmulti-bridge contact MBCA1. That is, the first multi-bridge contactMBCA1 may be a contact that may be electrically connected to the firstmulti-bridge channel structure MBCS1 and the second multi-bridge channelstructure MBCS2. The first multi-bridge contact MBCA1 may beelectrically connected to the third gate structure GS3 via a gatecontact GC2.

The third multi-bridge channel structure MBCS3 may be electricallyconnected to the fourth multi-bridge channel structure MBCS4 via asecond multi-bridge contact MBCA2. That is, the second multi-bridgecontact MBCA2 may be a contact that may be electrically connected to thethird multi-bridge channel structure MBCS3 and the fourth multi-bridgechannel structure MBCS4. The second multi-bridge contact MBCA2 may beelectrically connected to the first gate structure GS1 via a gatecontact GC3. The second gate structure GS2 and the fourth gate structureGS4 may be respectively connected to a word line W/L via gate contactsGC1 and GC4.

The first to fourth multi-bridge channel structures MBCS1, MBCS2, MBCS3and MBCS4, which are respectively surrounded by the first to fourth gatestructures GS1, GS2, GS3 and GS4, may include a plurality ofnano-bridges that may function as channels and may be stacked apart fromone another in a third direction (e.g., the Z direction) that issubstantially perpendicular to a plane defined by the first directionand the second direction to thereby improve electrical characteristics.

In other words, as described above, the first and second loadtransistors PU1 and PU2, the first and second drive transistors PD1 andPD2, and the first and second transfer transistors PG1 and PG2, whichare embodied in the first to fourth multi-bridge channel structuresMBCS1, MBCS2, MBCS3 and MBCS4, may include nano-bridges that serve asthe channels to provide improved electrical characteristics.

Furthermore, at least one of the first to fourth multi-bridge channelstructures MBCS1, MBCS2, MBCS3 and MBCS4, which are respectivelysurrounded by the first to fourth gate structures GS1, GS2, GS3 and GS4,may include a number of nano-bridges that is different from the numberof nano-bridges of the other multi-bride channel structures.

As described above, the number of stacked nano-bridges of at least oneof the first and second load transistors PU1 and PU2, the first andsecond drive transistors PD1 and PD2, and the first and second transfertransistors PG1 and PG2, which are embodied in the first to fourthmulti-bridge channel structures MBCS1, MBCS2, MBCS3 and MBCS4, may bedifferent from the number of stacked nano-bridges of the othertransistors.

Hereinafter, transistors having multi-bridge channels, which may formfirst and second load transistors PU1 and PU2, first and second drivetransistors PD1 and PD2, and first and second transfer transistors PG1and PG2, will be described. Also, multi-bridge channels, which areembodied in the first to fourth multi-bridge channel structures MBCS1,MBCS2, MBCS3 and MBCS4, and nano-bridges included in the multi-bridgechannels will be described.

FIG. 4 depicts a perspective view of transistors 200 having multi-bridgechannels used for an SRAM device according to an embodiment. FIG. 5depicts a cross-sectional view taken along a line XA-XA′ of FIG. 4, andFIG. 6 depicts a cross-sectional view taken along lines YA-YA′ andYB-YB′ of FIG. 4.

Specifically, the transistors 200 of FIGS. 4 to 6 may include a firsttransistor 200A and a second transistor 200B. The transistors 200 may beMOS transistors. FIG. 5 depicts a cross-sectional view of thetransistors 200, which is taken along the X direction of themulti-bridge channel structures MBCS1, MBCS2, MBCS3 and MBCS4 that aresurrounded by the gate structures GS1, GS2, GS3 and GS4 of FIG. 3.

FIG. 6 depicts a cross-sectional view of the transistors 200, which istaken along the Y direction of the multi-bridge channel structuresMBCS1, MBCS2, MBCS3 and MBCS4 that are surrounded by the gate structuresGS1, GS2, GS3 and GS4 of FIG. 3. In FIGS. 4 to 6, only two transistors200 are illustrated for brevity.

The transistors 200 may include multi-bridge channel structures MBCSaand MBCSb and gate structures GSa and GSb, which are formed on asemiconductor substrate 100. The multi-bridge channel structures MBCSaand MBCSb may respectively include multi-bridge channels MBCa and MBCb,and source and drain regions S and D.

The gate structures GSa and GSb may surround the multi-bridge channelsMBCa and MBCb. The gate structures GSa and GSb may surround themulti-bridge channels MBCa and MBCb, but not the source and drainregions S and D. Each of the gate structures GSa and GSb may include agate insulating layer 126 and a gate electrode 128.

The multi-bridge channels MBCa and MBCb of the transistors 200 mayinclude nano-bridges 112, 114, 116, 118, 120, 122 and 124, which arestacked on a semiconductor substrate 100 and spaced apart from oneanother. The nano-bridges 112, 114, 116, 118, 120, 122 and 124 may be achannels serving as charge paths between the source region S and thedrain region D.

For example, since the multi-bridge channels MBCa of the firsttransistor 200A include five nano-bridges 112, 114, 116, 118 and 120that are stacked, the first transistor 200A may have five multi-bridgechannels MBCa. Although the first transistor 200A is depicted as havingfive multi-bridge channels MBCa, the first transistor 200 a may includefewer multi-bridge channels, such as four multi-bridge channels. Inanother embodiment, the first transistor 200A may include more than fivemulti-bridge channels.

Since each of the multi-bridge channels MBCb of the second transistor200B includes two nano-bridges 122 and 124 that are stacked, the secondtransistor 200B may include two multi-bridge channels MBCb. Although thesecond transistor 200B is depicted as having two multi-bridge channelsMBCb, the second transistor 200 b may include fewer multi-bridgechannels, such as one multi-bridge channel. In another embodiment, thesecond transistor 200B may have more than two multi-bridge channels.

Each of the nano-bridges 112, 114, 116, 118, 120, 122 and 124 and thegate insulating layer 126 included in each of the gate stack structuresGSa and GSb may have a substantially quadrangular sectional shape. Whenthe nano-bridges 112, 114, 116, 118, 120, 122 and 124 have substantiallyquadrangular sectional shapes, the nano-bridges 112, 114, 116, 118, 120,122 and 124 may be referred to as nano-sheets.

The transistors 200 may be used as the first and second load transistorsPU1 and PU2, the first and second drive transistors PD1 and PD2, and/orthe first and second transfer transistors PG1 and PG2 of the SRAM device10 of FIGS. 1 to 3. In other words, if the transistors 200 are used inthe SRAM device 10 of FIGS. 1 to 3, at least one of the first and secondload transistors PU1 and PU2, the first and second drive transistors PD1and PD2, and the first and second transfer transistors PG1 and PG2 mayinclude transistors having a number of multi-bridge channels (e.g., MBCaand MBCb) that is different from the number of multi-bridge channels ofthe other transistors.

In an embodiment, the first transistor 200A may be used as the first andsecond drive transistors PD1 and PD2 and the first and second transfertransistors PG1 and PG2 of the SRAM device 10 of FIGS. 1 to 3. In otherwords, the first and second drive transistors PD1 and PD2 may includethe same number of multi-bridge channels MBCa as the first and secondtransfer transistors PG1 and PG2.

The second transistor 200B may be used as the first and second loadtransistors PU1 and PU2 of the SRAM device 10 of FIGS. 1 to 3. In otherwords, the number of multi-bridge channels MBCb of the first and secondload transistors PU1 and PU2 may be less than the number of multi-bridgechannels MBCa of the first and second drive transistors PD1 and PD2 andthe first and second transfer transistors PG1 and PG2.

For example, the number of multi-bridge channels MBCb of the first andsecond load transistors PU1 and PU2 may be about 70% or less than thenumber of multi-bridge channels MBCa of the first and second drivetransistors PD1 and PD2 and the first and second transfer transistorsPG1 and PG2.

When the number of multi-bridge channels MBCa of the first and secondload transistors PU1 and PU2 is less than the number of multi-bridgechannels MBCb of the first and second drive transistors PD1 and PD2 andthe first and second transfer transistors PG1 and PG2, write-operationcharacteristics of the SRAM device may be improved.

More specifically, a write operation may include writing a high electricpotential H to nodes (a first node or a second node) that are connectedto the first and second load transistors PU1 and PU2 and transferringcharges into the first and second transfer transistors PG1 and PG2. Inthis case, when a large quantity of charge flows into the nodes (thefirst node or the second node) that are connected to the first andsecond load transistors PU1 and PU2, a transition to a low electricpotential L may be retarded and potentially enabling a write failure.

Thus, if the quantity of charge transferred into the first and secondtransistors PG1 and PG2 is reduced by reducing the number ofmulti-bridge channels of the first and second load transistors PU1 andPU2 to be less than the number of multi-bridge channels of the first andsecond drive transistors PD1 and PD2 or the first and second transfertransistors PG1 and PG2, write-operation characteristics of the SRAMdevice may be improved.

FIGS. 7A and 7B depict cross-sectional views of transistors 200-1 havingmulti-bridge channels that is used for an SRAM device according to anembodiment.

Specifically, FIGS. 7A and 7B respectively depict cross-sectional viewscorresponding to FIGS. 4 and 5. FIG. 7B depicts a cross-sectional viewtaken along a line XA-XA′ of FIG. 7A. The transistors 200-1 may be thesame as the transistors 200 of FIGS. 4 and 5 except for sectional shapesof nano-bridges 112, 114, 116, 118, 120, 122 and 124 that are includedin multi-bridge channels MBCa and MBCb, and a gate insulating layer 126that is included in each of gate stack structures GSa and GSb.

In FIGS. 7A and 7B, the same reference numerals are used to denote thesame elements as in FIGS. 4 and 5, and repeated descriptions thereofwill be omitted or simplified for brevity. The transistors 200-1 mayinclude a first transistor 200A-1 and a second transistor 200B-1. Unlikethe transistors 200 of FIGS. 4 and 5, in the transistors 200-1 of FIGS.7A and 7B, the nano-bridges 112, 114, 116, 118, 120, 122 and 124 thatare included in the multi-bridge channels MBCa and MBCb, and the gateinsulating layer 126 that is included in each of gate stack structuresGSa and GSb may have substantially circular sectional shapes.

When the nano-bridges 112, 114, 116, 118, 120, 122 and 124 havesubstantially circular sectional shapes, the nano-bridges 112, 114, 116,118, 120, 122 and 124 may be referred to as nano-wires.

FIG. 8 depicts a cross-sectional view of transistors 200-2 havingmulti-bridge channels that is used for an SRAM device according to anembodiment.

Specifically, FIG. 8 depicts a cross-sectional view corresponding toFIG. 5. FIG. 7B is a cross-sectional view taken along a line XA-XA′ ofFIG. 7A. The transistors 200-2 may be the same as the transistors 200 ofFIG. 5 except for sectional shapes of nano-bridges 112, 114, 116, 118,120, 122 and 124 that are included in multi-bridge channels MBCa andMBCb, and the gate insulating layer 126 that is included in each of gatestack structures GSa and GSb. In FIG. 8, the same reference numerals areused to denote the same elements as in FIGS. 4 to 6, and repeateddescriptions thereof will be omitted or simplified for brevity.

The transistors 200-2 may include a first transistor 200A-2 and a secondtransistor 200B-2. Unlike the transistors 200 of FIG. 5, in thetransistors 200-2, the nano-bridges 112, 114, 116, 118, 120, 122 and 124included in multi-bridge channels MBCa and MBCb, and the gate insulatinglayer 126 included in each of the gate stack structures GSa and GSb mayhave substantially rectangular sectional shapes.

When the nano-bridges 112, 114, 116, 118, 120, 122 and 124 havesubstantially rectangular sectional shapes, the nano-bridges 112, 114,116, 118, 120, 122 and 124 may be referred to as nano-sheets.

FIGS. 9 and 10 depict cross-sectional views of transistors 200-3 and200-4 having multi-bridge channels that are used for an SRAM deviceaccording to an embodiment.

Specifically, FIGS. 9 and 10 depict cross-sectional views correspondingto FIGS. 5 and 8. The transistors 200-3 and 200-4 may be the same as thetransistors 200 and 200-2 of FIGS. 5 and 8 except that the transistors200-3 and 200-4 include third transistors 200C and 200C-1. In FIGS. 9and 10, the same reference numerals are used to denote the same elementsas in FIGS. 4 to 8, and repeated descriptions thereof will be omitted orsimplified for brevity.

The transistors 200-3 and 200-4 may include nano-bridges 112, 114, 116,118, 120, 122 and 124 that are included in multi-bridge channels MBCa,MBCb, and MBCc and gate stack structures GSa, GSb and GSc. Thetransistors 200-3 and 200-4 may include first transistors 200A and200A-2, second transistors 200B and 200B-2 and third transistors 200Cand 200C-1.

The first transistors 200A and 200A-2 may include five nano-bridges 112,114, 116, 118 and 120, and the first transistors 200A and 200A-2 mayinclude five multi-bridge channels MBCa. The second transistors 200B and200B-2 may include two nano-bridges 112 and 114, and the secondtransistors 200B and 200B-2 may include two multi-bridge channels MBCb.

The third transistors 200C and 200C-1 include fourth nano-bridges 112,114, 116 and 118, and the third transistors 200C and 200C-1 may includefour multi-bridge channels MBCc. Although the third transistors 200C and200C-1 are depicted as including fourth multi-bridge channels MBCc, thethird transistors 200C and 200-C may include a fewer number ofmulti-bridge channels (e.g., three multi-bridge channels) than thenumber of multi-bridge channels included in the first transistors 200Aand 200A-2.

The transistors 200-3 and 200-4 may be used as the first and second loadtransistors PU1 and PU2, the first and second drive transistors PD1 andPD2, and/or the first and second transfer transistors PG1 and PG2 of theSRAM device 10 of FIGS. 1 to 3.

In other words, if the transistors 200-3 and 200-4 are used in the SRAMdevice 10 of FIGS. 1 to 3, at least one of the first and second loadtransistors PU1 and PU2, the first and second drive transistors PD1 andPD2 and the first and second transfer transistors PG1 and PG2 mayinclude a transistor having a number of multi-bridge channels MBCa,MBCb, and MBCc that is different from the number of multi-bridgechannels of the other transistors.

In an embodiment, the first transistor 200A-1 may be used as the firstand second drive transistors PD1 and PD2 of the SRAM device 10 of FIGS.1 to 3. The second transistor 200B-1 may be used as the first and secondload transistors PU1 and PU2 of the SRAM device 200 of FIGS. 1 to 3. Thethird transistors 200C and 200C-1 may be used as the first and secondtransfer transistors PG1 and PG2 of the SRAM device 10 of FIGS. 1 to 3.

In other words, the number of multi-bridge channels MBCc of the firstand second transfer transistors PG1 and PG2 may be less than the numberof multi-bridge channels MBCa of the first and second drive transistorsPD1 and PD2. If the number of the multi-bridge channels MBCc of thefirst and second transfer transistors PG1 and PG2 is less than thenumber of multi-bridge channels MBCa of the first and second drivetransistors PD1 and PD2, the SRAM device may be prevented fromexhibiting disturbance failures, and, therefore, may operate at a lowoperating voltage.

More specifically, if a current is supplied to the first and secondtransfer transistors PG1 and PG2 that is less than a current supplied tothe first and second drive transistors PD1 and PD2 by reducing thenumber of the multi-bridge channels MBCc of the first and secondtransfer transistors PG1 and PG2, a node voltage between the first andsecond transfer transistors PG1 and PG2, and the first and second drivetransistors PD1 and PD2 may be maintained low. In this case, adisturbance failure of the SRAM device may be minimized in which“disturbance failure” refers to a situation in which the opposite latchcircuit is turned on due to a rise in node voltage caused by noise.

If the number of multi-bridge channels MBCc of the first and secondtransfer transistors PG1 and PG2 is less than the number of multi-bridgechannels MBCa of the first and second drive transistors PD1 and PD2, thecurrents of the first and second transfer transistors PG1 and PG2 of theSRAM device may be effectively reduced. Thus, disturbancecharacteristics of the SRAM device may be improved, and the SRAM devicemay operate even at a relatively low operating voltage.

FIGS. 11A to 20A and 11B to 20B depict diagrams of stages of a method ofmanufacturing transistors according to an embodiment. FIG. 21 is a flowdiagram 2100 of a method of manufacturing transistors according to anembodiment.

Specifically, FIGS. 11A to 20A and 11B to 20B will be presented todescribe the method of manufacturing transistors 200 and 200-1. Althoughthe transistors 200 and 200-1 may be manufactured in various ways, FIGS.11A to 20A and 11B to 20B depict stages of a method of manufacturing thetransistors 200 and 200-1 according to one embodiment.

FIGS. 11A to 20A depict perspective views of stages of a method ofmanufacturing transistors according to an embodiment. FIGS. 11B to 20Brespectively depict cross-sectional views taken along lines XA-XA′ ofFIGS. 11A to 20A.

Referring to FIGS. 11A and 11B, a semiconductor substrate 100 may beprepared. The semiconductor substrate 100 may be a bulk siliconsubstrate or a silicon-on-insulator (SOI) substrate. Also, one or moredevice isolation regions (not shown) may be formed in the semiconductorsubstrate 100. A device isolation region may be formed by using anordinary process, such as a shallow trench isolation (STI) process.

Subsequently, as shown in operation 2101 of FIG. 21, channel-formingpreliminary layers 310 and 320 may be formed on a semiconductorsubstrate 100. The channel-forming preliminary layer 310 may correspondto the first transistor (refer to 200A in FIGS. 4 to 6) having fivemulti-bridge channels MBCa. The channel-forming preliminary layer 320may correspond to the second transistor (refer to 200B in FIGS. 4 to 6)having two multi-bridge channels MBCb.

The channel-forming preliminary layer 310 may be formed by sequentiallystacking a first sacrificial layer 111 a, a first channel layer 112 a, asecond sacrificial layer 113 a, a second channel layer 114 a, a thirdsacrificial layer 115 a, a third channel layer 116 a, a fourthsacrificial layer 117 a, a fourth channel layer 118 a, a fifthsacrificial layer 119 a and a fifth channel layer 120 a.

The channel-forming preliminary layer 320 may be formed by sequentiallystacking a first sacrificial layer 121 a, a first channel layer 122 a, asecond sacrificial layer 123 a, a second channel layer 124 a and a thirdsacrificial layer 125 a. The first sacrificial layers 111 a and 121 a,the first channel layers 112 a and 122 a, the second sacrificial layers113 a and 123 a, and the second channel layers 114 a and 124 a of thechannel-forming preliminary layers 310 and 320 may be formed during thesame process operation. The number of multi-bridge channels may becontrolled by varying the numbers of sacrificial layers and channellayers included in the channel-forming preliminary layers 310 and 320.

The channel-forming preliminary layers 310 and 320 may be formed byusing an epitaxial growth method or a molecular beam epitaxy method. Thesacrificial layers 111 a, 113 a, 115 a, 117 a, 119 a, 121 a, 123 a and125 a may respectively include material layers having similar latticeconstants as the channel layers 112 a, 114 a, 116 a, 118 a, 120 a, 122 aand 124 a, and may respectively have etch selectivities with respect tothe channel layers 112 a, 114 a, 116 a, 118 a, 120 a, 122 a and 124 a.

For example, when the channel layers 112 a, 114 a, 116 a, 118 a, 120 a,122 a and 124 a are formed by using an epitaxial silicon layer, thesacrificial layers 111 a, 113 a, 115 a, 117 a, 119 a, 121 a, 123 a and125 a may be formed by using an epitaxial silicon germanium layer. Inthis case, the sacrificial layers 111 a, 113 a, 115 a, 117 a, 119 a, 121a, 123 a and 125 a, and the channel layers 112 a, 114 a, 116 a, 118 a,120 a, 122 a and 124 a may be continuously formed in-situ.

The sacrificial layers 111 a, 113 a, 115 a, 117 a, 119 a, 121 a, 123 aand 125 a, and the channel layers 112 a, 114 a, 116 a, 118 a, 120 a, 122a and 124 a included in the channel-forming preliminary layers 310 and320 may have a thickness of about 10 nm to about 30 nm. The sacrificiallayer 125 a included in the channel-forming preliminary layer 310 may beformed to have a greater thickness than other sacrificial layers toprevent occurrence of a step difference between the first transistor(refer to 200A in FIGS. 4 to 6) and the second transistor (refer to 200Bin FIGS. 4 to 6).

A mask layer 330 may be formed on the channel-forming preliminary layers310 and 320. The mask layer 330 may include a material having a highetch selectivity with respect to silicon and silicon germanium. The masklayer 330 may include silicon nitride in view of subsequent processes.The mask layer 330 may be formed to a thickness of, for example, about100 nm or less. The mask layer 330 may be formed by using an ordinarydeposition process, for example, a chemical vapor deposition (CVD)process, a low-pressure CVD (LPCVD) process, or a plasma-enhanced CVD(PECVD) process.

Referring to FIGS. 12A and 12B and operation 2102 of FIG. 21, thechannel-forming preliminary layers 310 and 320 and the mask layer 330may be patterned, thereby forming channel-forming preliminary patterns310 a and 320 a and a mask pattern 330 a. The channel-formingpreliminary patterns 310 a and 320 a may include patterned sacrificiallayers 111 b, 113 b, 115 b, 117 b, 119 b, 121 b, 123 b and 125 b, andpatterned channel layers 112 b, 114 b, 116 b, 118 b, 120 b, 122 b and124 b. The mask pattern 330 a may be formed to have a smaller width inthe X-direction and a smaller length in the Y-direction than a width inthe X-direction and a width in the Y-direction of the channel-formingpreliminary patterns 310 a and 320 a.

The channel-forming preliminary patterns 310 a and 320 a and the maskpattern 330 a may be formed by using the following method. Thechannel-forming preliminary layers 310 and 320 and the mask layer 330may be patterned by using a photolithography process according to sizesof the channel-forming preliminary patterns 310 a and 320 a. Also, thepatterned mask layer 330 may be further etched by using anisotropic-etching process to form the mask pattern 330 a having asmaller width in the X-direction and a smaller length in the Y-directionthan the channel-forming preliminary patterns 310 a and 320 a.

Referring to FIGS. 13A and 13B and operation 2103 of FIG. 21, a molderinsulating layer may be deposited to have a thickness that covers thesemiconductor substrate 100, the channel-forming preliminary patterns310 a and 320 a, and the mask pattern 330 a, and planarized until themask pattern 330 a is exposed. As a result, a molder pattern 332 may beformed on the semiconductor substrate 100 and surround thechannel-forming preliminary patterns 310 a and 320 a and the maskpattern 330 a.

The molder pattern 332 may include a material having a high etchselectivity with respect to the mask pattern 330 a, the sacrificiallayers 111 b, 113 b, 115 b, 117 b, 119 b, 121 b, 123 b and 125 b, andthe channel layers 112 b, 114 b, 116 b, 118 b, 120 b, 122 b and 124 b.For example, if the mask pattern 330 a includes silicon nitride, themolder pattern 332 may include silicon oxide. In this case, the molderpattern 332 may include a silicon oxide layer selected from the groupconsisting of an undoped silicate glass (USG) layer, a high-densityplasma (HDP) oxide layer, a PE-TEOS layer and a combination thereof.

Referring to FIGS. 14A and 14B and operation 2104 of FIG. 21, the molderpattern 332 and the mask pattern 330 a may be simultaneously patterned,thereby forming a dummy gate pattern 340 including a portion 332 b ofthe molder pattern 332 and a portion 330 b of the mask pattern 330 a. Aportion 332 a of the molder pattern 332 may surround the channel-formingpreliminary patterns 310 a and 320 a. The patterning process may beperformed by using a photoresist pattern as an etch mask.

Additionally, the etching of the molder pattern 332 and the mask pattern330 a may be performed until top surfaces of the channel-formingpreliminary patterns 310 a and 320 a are exposed on both sides of thedummy gate pattern 340. The dummy gate pattern 340, which is formed as aresult of the etching process, may be a line-type pattern extending inan X direction.

Referring to FIGS. 15A and 15B and operation 2105 of FIG. 21, thechannel-forming preliminary patterns 310 a and 320 a may beanisotropically dry etched by using the dummy gate pattern 340 as anetch mask. In this case, an appropriate etch gas may be selected to usethe dummy gate pattern 340 and the remaining molder pattern 332 a as anetch mask. For example, the etching of the molder pattern 332 and themask pattern 330 a may be performed by using an etch gas having the sameetch selectivity with respect to silicon and silicon germanium and highetch selectivities with respect to a silicon oxide layer and a siliconnitride layer.

In addition, the sacrificial layers 111 b, 113 b, 115 b, 117 b, 119 b,121 b, 123 b and 125 b, and the channel layers 112 b, 114 b, 116 b, 118b, 120 b, 122 b and 124 b may be continuously etched in-situ. As theresult of the etching process, channel-forming preliminary patterns 310b and 320 b may remain only under the dummy gate pattern 340.

A pair of first holes 344 may be formed on both sides of the remainingchannel-forming preliminary patterns 310 b and 320 b and defined by theremaining molder pattern 332 a and the remaining channel-formingpreliminary patterns 310 b and 320 b. Portions of the top surface of thesemiconductor substrate 100 may be exposed by the pair of first holes344. Due to the patterning process, the channel-forming preliminarypatterns 310 b and 320 b may include the sacrificial layers 111 c, 113c, 115 c, 117 c, 119 c, 121 c, 123 c and 125 c, and the channel layers112 c, 114 c, 116 c, 118 c, 120 c, 122 c and 124 c.

Referring to FIGS. 16A and 16B and operation 2106 of FIG. 21, source anddrain patterns 346 may be formed in the first holes 344. The source anddrain patterns 346 may include single-crystalline silicon or polysilicon(poly-Si). When the source and drain patterns 346 are formed by using asilicon epitaxial layer (or epi-layer), the first holes 344 may befilled with single-crystalline silicon by using a selective epitaxialgrowth (SEG) process of selectively forming a silicon epi-layer only onthe portions of the top surface of the semiconductor substrate 100exposed by the pair of first holes 344.

The single crystalline silicon layer or the poly-Si layer deposited tofill the first holes 344 may also be planarized by using an etchbackprocess until a top surface of the portion 332 a of the molder pattern332 is exposed. As a result, the source and drain patterns 346 may havea height that is substantially equal to a level of the top surfaces ofthe remaining channel-forming preliminary patterns 310 a and 320 a.

Referring to FIGS. 17A and 17B and operation 2107 of FIG. 21, a bufferinsulating layer may be deposited to a thickness on the remaining molderpattern 332 a that covers the source and drain patterns 346, and thedummy gate pattern 340, and planarized until the dummy gate pattern 340is exposed. As a result, a buffer layer pattern 348 may be formed on theremaining molder pattern 332 a and source and drain patterns 346. Thebuffer layer pattern 348 may include the same material as the molderpattern 332 a.

Thereafter, as shown in FIG. 17A, only the remaining mask pattern 330 bmay be removed from the dummy gate pattern 340. As a result, topsurfaces of the channel-forming preliminary patterns 310 b and 320 b,and a buffer layer pattern 348 may be formed in a space occupied by themask pattern 330 b and surrounded with the buffer layer pattern 348 andthe portion 332 b of the molder pattern 332 that is included in thedummy gate pattern 340.

Subsequently, the remaining channel-forming preliminary patterns 310 band 320 b exposed by the groove 350 may be anisotropically etched. Inthis case, an appropriate etch gas may be selected to use the portion332 b of the molder pattern 332 b included in the dummy gate pattern 340and the buffer layer pattern 348 as an etch mask. For example, theetching process may be performed by using an etch gas having the sameetch selectivity with respect to silicon and silicon germanium and ahigh etch selectivity with respect to silicon oxide.

As a result of the etching process, channel-forming preliminary patterns310 c and 320 c may remain only under the portion 332 b of the molderpattern 332 included in the dummy gate pattern 340. Due to the etchingprocess, the channel-forming preliminary patterns 310 c and 320 c mayinclude sacrificial layers 111 d, 113 d, 115 d, 117 d, 119 d, 121 d, 123d and 125 d, and channel layers 112 d, 114 d, 116 d, 118 d, 120 d, 122 dand 124 d.

A second hole 352 may be formed in a space defined by the remainingchannel-forming preliminary patterns 310 c and 320 c and the source anddrain patterns 346 and connected to the groove 350. The top surface ofthe semiconductor substrate 100 may be exposed by the second hole 352.

Subsequently, as shown in FIG. 17B, a channel formation blocking layer354 may be formed in the semiconductor substrate 100 as needed. Sincethe channel formation blocking layer 354 is an arbitrary element, thepresent process may also be arbitrary. The channel formation blockinglayer 354 may be formed by implanting ions into the semiconductorsubstrate 100 exposed by the groove 350 and the second hole 352. In thiscase, the portion 332 of the molder pattern 332 b of the dummy gatepattern 340 and the buffer layer pattern 348 may be used as a mask.

Since the channel formation blocking layer 354 is used to prevent anoperation of a base transistor, ions of the same conductivity type asthe semiconductor substrate 100 may be implanted. For example, when thesemiconductor substrate 100 is of a p-type, a Group 3B element, such asboron (B) or indium (In), may be implanted.

Referring to FIGS. 18A and 18B and operation 2108 of FIG. 2108, to beginwith only the buffer layer pattern 348 and the remaining portions 332 aand 332 b of the molder pattern 332 may be removed by using a selectiveetching process. The etching process may be performed by using a siliconoxide etch gas or silicon oxide etchant having a high selectivity withrespect to silicon and/or silicon germanium.

Thereafter, the sacrificial layer 111 d, 113 d, 115 d, 117 d, 119 d, 121d, 123 d and 125 d of the channel-forming preliminary patterns 310 c and320 c may be removed. As a result, only the source and drain patterns346 and the channel layer patterns 112 d, 114 d, 116 d, 118 d, 120 d,122 d and 124 d may remain on the semiconductor substrate 100. Thechannel layer patterns 112 d, 114 d, 116 d, 118 d, 120 d, 122 d and 124d may be between the source and drain patterns 346 and spaced apart fromone another.

The remaining channel layer patterns 112 d, 114 d, 116 d, 118 d, 120 d,122 d and 124 d may remain between the source and drain patterns 346over the semiconductor substrate 100. The channel layer patterns 112 d,114 d, 116 d, 118 d, 120 d, 122 d and 124 d may have quadrangularsectional shapes. The channel layer patterns 112 d, 114 d, 116 d, 118 d,120 d, 122 d and 124 d may be nano-bridges, which may be included in themulti-bridge channels described above with reference to FIGS. 4 to 10.

Referring to FIGS. 19A and 19B and operation 2109 of FIG. 21, thesemiconductor substrate 100 on which the source and drain patterns 346and the plurality of channel layer patterns 112 d, 114 d, 116 d, 118 d,120 d, 122 d and 124 d are formed may be annealed by a first annealingprocess.

The first annealing process may be a process that tends to make thesectional shapes of the channel layer patterns 112 d, 114 d, 116 d, 118d, 120 d, 122 d and 124 d round. In other words, the first annealingprocess may not be performed when it is not intended to round thesectional shapes of the channel layer patterns 112 d, 114 d, 116 d, 118d, 120 d, 122 d and 124 d. If the channel layer patterns 112 d, 114 d,116 d, 118 d, 120 d, 122 d and 124 d (or nano-bridges) havesubstantially circular sectional shapes or substantially ellipticalcircular shapes, more ideal isotropic electric potentials for channelsmay be formed than if the channel layer patterns 112 d, 114 d, 116 d,118 d, 120 d, 122 d and 124 d (or nano-bridges) have substantiallyquadrangular sectional shapes.

The first annealing process may be performed at such an appropriatetemperature that sectional shapes of the channel layer patterns 112 d,114 d, 116 d, 118 d, 120 d, 122 d and 124 d become round. For example,the first annealing process may be performed in a hydrogen atmosphere ata temperature of about 600° C. to about 1200° C. Alternatively, thefirst annealing process may be performed in an argon atmosphere at atemperature of about 900° C. to about 1200° C.

Referring to FIGS. 20A and 20B and operation 2110 of FIG. 21, theresultant structure may be annealed a second time in an oxygenatmosphere or an ozone atmosphere to form a gate insulating layer 126.When the resultant structure is annealed in the oxygen or ozoneatmosphere, an exposed silicon surface may be consumed so that the gateinsulating layer 126 may be formed as a silicon oxide layer on theexposed silicon surface. An annealing temperature and time duration ofthe second annealing process may vary according to a desired thicknessof the gate insulating layer 126.

A gate electrode 128 may be formed between the source and drain patterns346. The gate electrode 128 may include a single poly-Si layer or acompound layer including a poly-Si layer and a conductive layer having alower resistivity than the poly-Si layer. Poly-Si may be deposited invacant spaces between the source and drain patterns 346 in which thechannel layer patterns 112 d, 114 d, 116 d, 118 d, 120 d, 122 d and 124d are formed.

The gate electrode 128 may be formed as a line type extendingsubstantially in an X′ direction. Subsequently, source and drain regions348, S, or D may be defined by performing an ion-implantation process onthe source and drain patterns 346. Thus, the formation of thetransistors 200 or 200-1 may be completed.

FIG. 22 depicts a block diagram of a logic device 800 including one ormore SRAM devices 200 according to an embodiment.

Specifically, the logic device 800 may include an SRAM forming region400 and a logic region 600. The SRAM forming region 400 may include oneor more SRAM devices 200 according to an embodiment. A single SRAMdevice 200 is depicted as an example in FIG. 22 for brevity.

As described above, the SRAM device 200 may include a first inverterincluding a first load transistor and a first drive transistor, a secondinverter including a second load transistor and a second drivetransistor, a first transfer transistor connected to an output node ofthe first inverter, and a second transfer transistor connected to anoutput node of the second inverter.

In addition, at least one of the first and second load transistors, thefirst and second drive transistors, and the first and second transfertransistors may include a transistor having a number of multi-bridgechannels that is different from the number of multi-bridge channels ofthe other transistors.

A circuit element configured to process data may be installed in thelogic region 600. A circuit element configured to process data of theSRAM device 200 or external data may be installed in the logic region600. For example, a MOS transistor 500 may be formed in the logic region600.

FIG. 23 depicts a block diagram of a card 1400 that includes one or moreSRAM devices according to an embodiment.

Specifically, the card 1400 may include a controller 1410 and a memory1420 located on a circuit board 1402. The controller 1410 and the memory1420 may be located to exchange electric signals. For example, when thecontroller 1410 issues a command, the memory 1420 may transmit data inresponse to the command. The memory 1420 or the controller 1410 mayinclude one or more SRAM devices according to an embodiment.

The card 1400 may be one of various kinds of cards, for example, amemory stick card, a smart media (SM) card, a secure digital (SD) card,a mini SD card, or a multimedia card (MMC).

FIG. 24 depicts a schematic block diagram of an electronic circuit board1500 including one or more SRAM devices according to an embodiment.

Specifically, the electronic circuit board 1500 may include amicroprocessor (MP) 1530 located on a circuit board 1525, a main storagecircuit 1535 and a supplementary storage circuit 1540 configured tocommunicate with the MP 1530, an input signal processing circuit 1545configured to issue a command to the MP 1530, an output signalprocessing circuit 1550 configured to receive the command from the inputsignal processing circuit 1545, and a communication signal processingcircuit 1555 configured to transmit and receive electric signals to andfrom other circuit boards. Arrows depicted in FIG. 23 may be interpretedas paths through which the electric signals are transmitted.

The MP 1530 may receive and process various electric signals, outputprocessing results, and control other elements of the electronic circuitboard 1500. The MP 1530 may be interpreted as, for example, a centralprocessing unit (CPU) and/or a main control unit (MCU).

The main storage circuit 1535 may temporarily store data, which isalways or frequently required by the MP 1530, or processed data or datato be processed. Since the main storage circuit 1535 requires a highresponse speed, the main storage circuit 1535 may include asemiconductor memory chip. More specifically, the main storage circuit1535 may be a semiconductor memory called a cache. The main storagecircuit 1535 may include one or more SRAM devices according to anembodiment. Furthermore, the main storage circuit 1535 may includedynamic random access memory (DRAM), resistive RAM (RRAM), appliedsemiconductor memories thereof (e.g., utilized RAM, ferroelectric RAM(FRAM), fast-cycle RAM, phase-change RAM (PRAM), and magnetic RAM(MRAM)), and/or other semiconductor memories.

Additionally, the main storage circuit 1535 may be independent ofvolatility and non-volatility and include a random access memory (RAM).The supplementary storage circuit 1540, which is a mass storage device,may be a non-volatile semiconductor memory (e.g., a flash memory), ahard disk drive (HDD) using a magnetic field, or a compact disc drive(CDD) using light. As compared with the main storage circuit 1535, thesupplementary storage circuit 1540 may be used when it is intended tostore mass data rather than to obtain a high operation speed. Thesupplementary storage circuit 1240 may be independent of random andnon-random and include a non-volatile storage device.

The supplementary storage circuit 1540 may include one or more SRAMdevices according to an embodiment. The input signal processing circuit1545 may convert an external command into an electric signal or transmitan external electric signal to the MP 1530.

The external command or the external electric signal may be an operationcommand, an electric signal to be processed, or data to be stored. Theinput signal processing circuit 1545 may be, for example, a terminalsignal processing circuit configured to process a signal transmittedfrom a keyboard, a mouse, a touch pad, an image recognizer, or varioussensors, an image signal processing circuit configured to process animage signal input to a scanner or a camera, or one of various sensorsor input signal interfaces. The input signal processing circuit 1545 mayinclude one or more SRAM devices according to an embodiment.

The output signal processing circuit 1550 may be an element configuredto externally transmit an electric signal processed by the MP 1530. Forexample, the output signal processing circuit 1550 may be a graphiccard, an image processor, an optical converter, a beam panel card, or amultifunctional circuit. The output signal processing circuit 1550 mayinclude one or more SRAM devices according to an embodiment.

The communication signal processing circuit 1555 may be an elementconfigured to directly transmit and receive electric signals to and fromother electronic systems or other circuit boards without passing throughthe input signal processing circuit 1545 or the output signal processingcircuit 1550. For instance, the communication circuit 1555 may be amodem of a PC system, a local area network (LAN) card, or one of variousinterface circuits. The communication circuit 1555 may include one ormore SRAM devices according to an embodiment.

FIG. 25 depicts a schematic block diagram of an electronic system 1600including one or more SRAM devices according to an embodiment.

Referring to FIG. 25, the electronic system 1600 may include a controlunit 1665, an input unit 1670, an output unit 1675, and a storage unit1680 and further include a communication unit 1685 and/or an additionaloperation unit 1690.

The control unit 1665 may generally control the electronic system 1600and respective portions. The control unit 1665 may be interpreted as acentral processing unit (CPU) or a central control unit (CCU) andinclude the electronic circuit board (refer to 1500 in FIG. 23)according to an embodiment. Also, the control unit 1665 may include anSRAM device according to an embodiment.

The input unit 1670 may transmit an electric command signal to thecontrol unit 1665. The input unit 1670 may be a keyboard, a keypad, amouse, a touch pad, an image recognizer (e.g., a scanner), or one ofvarious input sensors. The input unit 1670 may include one or more SRAMdevices according to an embodiment.

The output unit 1675 may receive the electric command signal from thecontrol unit 1665 and output a processing result of the electronicsystem 1600. The output unit 1675 may be a monitor, a printer, a beamirradiator, or one of various mechanical devices. The output unit 1675may include one or more SRAM devices according to an embodiment.

The storage unit 1680 may be an element configured to temporarily orpermanently store an electric signal to be processed by the control unit1665 or an electric signal already processed by the control unit 1665.The storage unit 1680 may be physically or electrically connected orcoupled to the control unit 1665. The storage unit 1680 may be asemiconductor memory, a magnetic storage device (e.g., a hard disk), anoptical storage device (e.g., a compact disc), or another server havinga data storage function. Also, the storage unit 1680 may include one ormore SRAM devices according to an embodiment.

The communication unit 1685 may receive an electric command signal fromthe control unit 1665 and transmit or receive an electric signal to orfrom another electronic system. The communication unit 1685 may be awired transceiver (e.g., a modem and a LAN card), a wireless transceiver(e.g., a WiBro interface), or an infrared (IR) port. Also, thecommunication unit 1685 may include one or more SRAM devices accordingto an embodiment.

The additional operation unit 1690 may perform a physical operation or amechanical operation in response to a command of the control unit 1665.For example, the operation unit 1690 may be an element (e.g., a plotter,an indicator, or an up/down operator) capable of a mechanical operation.The electronic system 1600 according to an embodiment may be a computer,a network server, a networking printer or scanner, a wirelesscontroller, a mobile communication terminal, an exchanger or one ofother electronic devices capable of programmed operations.

In addition, the electronic system 1600 may be used for a mobile phone,a MP3 player, navigation system, a portable multimedia player (PMP), asolid-state disk (SSD) or a household appliance.

FIG. 26 is a schematic diagram of an electronic system 1700 including anSRAM device according to an embodiment.

Specifically, the electronic system 1700 may include a controller 1710,an input/output (I/O) device 1720, a memory 1730, and an interface 1740.The electronic system 1700 may be a mobile system or a system configuredto transmit or receive information. The mobile system may be a personaldigital assistant (PDA), a portable computer, a web tablet, a wirelessphone, a mobile phone, a digital music player, or a memory card.

The controller 1710 may execute a program and control the electronicsystem 1700. The controller 1710 may include an SRAM device according toan embodiment. The controller 1710 may be, for example, an MP, a digitalsignal processor (DSP), a microprocessor (MC), or a device similarthereto.

The I/O device 1720 may be used to input or output data to and from theelectronic system 1700. The electronic system 1700 may be connected toan external apparatus (e.g., a personal computer (PC) or a network) byusing the I/O device 1720 and exchange data with the external apparatus.The I/O device 1720 may be, for example, a keypad, a keyboard, or adisplay device.

The memory 1730 may store codes and/or data for operating the controller1710 and/or data processed by the controller 1710. The memory 1730 mayinclude an SRAM device according to an embodiment. The interface 1740may be data transmission path between the electronic system 1700 andanother external apparatus. The controller 1710, the I/O device 1720,the memory 1730, and the interface 1740 may communicate with one anothervia a bus 1750.

For example, the electronic system 1700 may be used for a mobile phone,a MP3 player, a navigation apparatus, a portable multimedia player(PMP), a solid-state disk (SSD), or a household appliance.

FIG. 27 depicts a schematic perspective view of an electronic deviceincluding one or more SRAM devices according to an embodiment.

Specifically, FIG. 27 depicts a specific example of applying theelectronic system 1700 of FIG. 26 to a mobile phone 1800. The mobilephone 1800 may include a system-on chip (SoC). The SoC 1810 may includeone or more SRAM devices according to an embodiment. Since the mobilephone 1800 includes the SoC 1810 in which a relatively highly efficientmain function block may be disposed, the mobile phone 1800 may haverelatively high performance. Also, since the SoC 1810 has relativelyhigh performance on the same area, the mobile phone 1800 may have aminimized size and relatively high performance.

While this inventive concept has been particularly shown and describedwith reference to embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the inventiveconcept as defined by the appended claims. The embodiments should beconsidered in descriptive sense only and not for purposes of limitation.Therefore, the scope of the inventive concept is defined not by thedetailed description of the inventive concept, but by the appendedclaims, and all differences within the scope will be construed as beingincluded in the inventive concept.

What is claimed is:
 1. A memory device, comprising: first to fourthmulti-bridge channel structures arranged in a second direction andsequentially spaced apart from one another in a first direction that issubstantially perpendicular to the second direction; a first gatestructure arranged in the first direction, the first gate structuresurrounding the first and second multi-bridge channel structures; firstand second source and drain regions located in the first and secondmulti-bridge channel structures on respective sides of the first gatestructure; a second gate structure arranged in the first direction andspaced apart from the first gate structure in the second direction, thesecond gate structure surrounding the first multi-bridge channelstructure; third source and drain regions located in the firstmulti-bridge channel structure on respective sides of the second gatestructure; a third gate structure spaced apart from the first gatestructure in the second direction and spaced apart from the second gatestructure in the first direction, the third gate structure surroundingthe third and fourth multi-bridge channel structures; fifth source anddrain regions located in the third and fourth multi-bridge channelstructures on respective sides of the third gate structure; a fourthgate structure spaced apart from the first gate structure in the firstdirection, the fourth gate structure surrounding the fourth multi-bridgechannel structure; and sixth source and drain regions located in thefourth multi-bridge channel structure on respective sides of the fourthgate structure, wherein the first to fourth multi-bridge channelstructures are surrounded with the first to fourth gate structures,respectively, and each of the first to fourth multi-bridge channelstructures comprises a plurality of nano-bridges serving as channels,the plurality of nano-bridges stacked apart from one another in a thirddirection that is substantially perpendicular to a plane defined by thefirst direction and the second direction, and at least one of the firstto fourth multi-bridge channel structures respectively surrounded by thefirst to fourth gate structures comprises a different number ofnano-bridges from the other multi-bridge channel structures.
 2. Thememory device of claim 1, wherein a number of nano-bridges included ineach of the second and third multi-bridge channel structures surroundedby the first and third gate structures is less than a number ofnano-bridges included in each of the first and fourth multi-bridgechannel structures surrounded by the second and fourth gate structures.3. The memory device of claim 1, wherein a number of nano-bridgesincluded in each of the second and third multi-bridge channel structuressurrounded with the first and third gate structure is less than a numberof nano-bridges included in each of the first and fourth multi-bridgechannel structures surrounded by the first and third gate structures. 4.The memory device of claim 1, wherein the first and second multi-bridgestack structures, the first gate structure, and the first and secondsource and drain regions comprise a first drive transistor and a firstload transistor that is electrically connected to the first drivetransistor, the first multi-bridge stack structure, the second gatestructure, and the third source and drain regions comprise a firsttransfer transistor, the third and fourth multi-bridge stack structures,the third gate structure, and the fourth and fifth source/drain regionscomprise a second load transistor and a second drive transistor that iselectrically connected to the second load transistor, and the fourthmulti-bridge stack structure, the fourth gate structure, and the sixthsource and drain regions comprise a second transfer transistor.
 5. Amemory device, comprising: an SRAM forming region including an SRAMdevice and a logic region configured to process data, wherein the SRAMdevice comprises a first inverter including a first load transistor andfirst drive transistor, a second inverter including a second loadtransistor and a second drive transistor, a first transfer transistorconnected to an output node of the first inverter, and a second transfertransistor connected to an output node of the second inverter, and atleast one of the first and second load transistors, the first and seconddrive transistors, and the first and second transfer transistorsincludes a transistor having a different number of multi-bridge channelsfrom the other transistors.
 6. A static random access memory (SRAM)device, comprising: a latch circuit comprising a first inverter and asecond inverter, the first inverter comprising a first input node, afirst load transistor, a first drive transistor and a first output node,the second inverter comprising a second input node, a second loadtransistor, a second drive transistor and a second output node, thefirst output node being electrically connected to the second input nodeand the second output node being electrically connected to the firstinput node; a first transfer transistor connected to the first outputnode; and a second transfer transistor connected to the second outputnode, each of the first and second load transistors, the first andsecond drive transistors, and the first and second transfer transistorscomprising a multi-bridge channel transistor, and at least one of thefirst and second load transistors, the first and second drivetransistors, and the first and second transfer transistors comprises amulti-bridge channel transistor having a number of multi-bridge channelsthat is different from a number of multi-bridge channels of the othertransistors, wherein the number of multi-bridge channels included ineach of the first and second transfer transistors is less than thenumber of multi-bridge channels included in each of the first and seconddrive transistors.
 7. The SRAM device of claim 6, wherein the transistorcomprising the multi-bridge channels comprises: a plurality ofnano-bridges stacked apart from each other; and gate structuresrespectively surrounding the each of the plurality of nano-bridges, eachof the gate structures comprising a gate insulating layer and a gateelectrode, wherein the nano-bridges comprise nano-wires or nano-sheets.